A coal-fired boiler of the related art has a furnace having a hollow shape and installed in a vertical direction, and a plurality of combustion burners are disposed along a circumferential direction on the wall of the furnace. Further, in the coal-fired boiler, a flue is connected to an upper portion of the furnace, and a heat exchanger for producing steam is disposed in the flue. For this reason, the combustion burner injects a mixture of fuel and air into the furnace, whereby a flame is formed, and a combustion gas is generated and flows to the flue. Then, steam is produced by heating water flowing through the heat exchanger with flue gas generated by combustion. Further, an exhaust duct is connected to the flue, a denitration facility, an electric dust collector, a desulfurizer, and the like are provided in the exhaust duct, and a chimney is provided at a downstream end portion of the flue.
In such a coal-fired boiler, in the denitration facility disposed in the exhaust duct, a reducing agent such as ammonia is supplied to the flue, and the flue gas supplied with the reducing agent is caused to pass through a catalyst which promotes the reaction between nitrogen oxides and the reducing agent, whereby the nitrogen oxides in the flue gas are removed. In this case, in order to efficiently remove the nitrogen oxides in the flue gas with the catalyst, it is necessary to maintain the temperature of the flue gas at a temperature higher than or equal to a predetermined reaction temperature. For this reason, the coal-fired boiler is provided with a bypass flow path bypassing the heat exchanger (an economizer), and during low load operation, high-temperature flue gas is bypassed to the upstream side of a denitration catalyst by the bypass flow path, whereby the temperature of the flue gas is maintained at a temperature higher than or equal to the reaction temperature.
If the high-temperature flue gas is bypassed during the low load operation of the coal-fired boiler, the high-temperature flue gas is mixed with low-temperature flue gas flowing through the flue. However, since the temperature difference between the low-temperature flue gas and the high-temperature flue gas is large, the low-temperature flue gas and the high-temperature flue gas are not sufficiently mixed with each other here, and acidic ammonium sulfate is precipitated in the low-temperature flue gas, so that there is a concern that the denitration catalyst may be blocked.
In order to solve such a problem, a technique to provide a mixer at a joining portion between the flue and the bypass flow path has been proposed, and for example, there are gas mixing devices described in patent literatures described below. In the gas mixing device described in PTL 1, an inflow part causing another gas flow into a duct through which a gas flows is provided, a plurality of blowing flow paths are connected to the inflow part, and a blowing port of each of the blowing flow paths is disposed such that gas is blown along a gas flow in the duct. Further, in the gas mixing device described in PTL 2, a plurality of box members are disposed in a first duct, and a second duct is connected to the first duct so as to communicate with a divided inlet opening of each of the box members.